This invention relates to the field of transporting and storing spent nuclear fuel and specifically to transferring spent nuclear fuel from a spent nuclear fuel pool to a storage cask.
In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. In the commercial nuclear industry, fuel assemblies are typically an assemblage of long, hollow, zircaloy tubes filled with enriched uranium. Upon depletion and subsequent removal, spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, spent nuclear fuel emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times.
In defueling a nuclear reactor, the spent nuclear fuel is removed from the reactor and placed in a canister that is submerged in a spent nuclear fuel pool. The pool facilitates cooling of the spent nuclear fuel and provides radiation shielding in addition to that which is supplied by the canister. However, the canister alone does not provide adequate containment of the radiation. As such, a loaded canister cannot be removed or transported from the spent nuclear fuel pool without additional radiation shielding. Because it is preferable to store spent nuclear fuel in a “dry state,” the canister must eventually be removed from the spent nuclear fuel pool. As such, apparatus that provide additional radiation shielding during the transport and long-term storage of the spent nuclear fuel are necessary.
In state of the art facilities, this additional radiation shielding is achieved by placing the loaded canisters in large cylindrical containers called casks. There are two types of casks used in the industry today, storage casks and transfer casks. A transfer cask is used to transport canisters of spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel in the “dry state” for long periods of time. Both transfer casks and storage casks have a cavity adapted to receive a canister of spent nuclear fuel and are designed to shield the environment from the radiation emitted by the spent nuclear fuel.
Storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel, size and weight are often secondary considerations (if considered at all). As a result, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh approximately 150 tons and have a height greater than 15 ft. As such, a common problem associated with storage casks is that they are too heavy to be lifted by most nuclear power plant cranes. Another common problem is that storage casks are too large to be placed in spent nuclear fuel pools. Thus, in order to store a canister of spent nuclear fuel in a storage cask, the canister must be removed from the pool, prepared in a staging area, and transported to the storage cask. Adequate radiation shielding is needed throughout all stages of this transfer procedure.
Removal from the storage pool and transport of the loaded canister to the storage cask is facilitated by a transfer cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the storage pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor and lowered into the storage pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the spent nuclear fuel for storage in the “dry state.” Once in the staging area, the water contained in the canister is pumped out of the canister. This is called dewatering. Once dewatered, the spent nuclear fuel is dried using a suitable process such as vacuum drying. Once dry, the canister is back-filled with an inert gas such as helium. The canister is then sealed and the canister and the transfer cask are once again lifted by the plant's crane and transported to an open storage cask. The transfer cask is then placed atop the storage cask and the canister is lowered into the storage cask.
Because it is imperative that the loaded canister is not directly exposed to the environment during the step of lowering the canister from the transfer cask into the storage cask, transfer casks have bottoms that can be withdrawn so that that the canister can be lowered directly into the storage cask. In prior art transfer casks, a rectangular compartment is attached to the bottom of the transfer cask. Within this rectangular compartment are two retractable sliding plates. When closed, these retractable plates act as the floor of the transfer cask's cavity on which the loaded canister rests. When fully retracted, the retractable plates leave an unobstructed path leading from the transfer cask to the storage cask through which the canister can be lowered. While the retractable plates and rectangular compartment provide radiation shielding for the canister as it passes between the transfer cask and the storage cask, this transfer cask design and transfer procedure have a number of deficiencies.
First off, it should be noted that the external surface of a loaded canister is in continuous contact with the ambient air after it is placed in a storage cask. Thus, it is desirable that the external surface of the canister remain free of any radioactive contamination. However, because it is virtually impossible to seal the retractable plates because of the hardware (rollers, tracks, etc.) required to make the plates retractable, the retractable plates of prior art transfer casks are ineffective in preventing the intrusion of pool water (which may contain radioactive particulates in emulsion) into the space between the canister's external surface and the walls of the transfer cask cavity. As such, the external surface of the canister can become contaminated. In order to deal with this threat of contamination, power plants employ a variety of measures such as continuously flushing the space with clean water from an external source. Such measures greatly complicate the process of fuel loading in the pool, leading to additional fuel loading time, added cost, and added risk to the operations staff who must work above the pool.
Second, as mentioned above, the transfer of the canister from the transfer cask to the storage cask occurs in a configuration where the transfer cask is stacked atop the storage cask. Because of the size of the transfer cask and storage cask, this stack can be quite tall, reaching heights of over thirty-five feet. Therefore, physical stability is a matter of concern, especially if a seismic event were to occur. As such, it is preferable to secure the transfer cask and the storage cask together to make the stack more robust. However, the presence of the retractable plate assembly at the bottom of the transfer cask precludes the design opportunity to configure a fastening detail. As a result, prior art transfer cask designs result in the undesirable situation where the transfer cask and the storage cask are stacked without being physically unconnected to each other.
Third, the retractable door assembly (including the retractable plates and the rectangular compartment) is quite heavy, reaching weights in excess of 12,000 lbs. As such, the area where radiation shielding is most needed, namely the cylindrical body of the transfer cask, must be made lighter to accommodate the heavy bottom region in order to remain within the lifting capacity of the power plant crane. Because the amount of radiation shielding provided by the transfer cask's cylindrical body is directly proportional to its weight, the heavy retractable door assembly results in a reduced amount of radiation shielding.
Fourth, the hardware of the retractable door assembly, such as the rollers and tracks, require lubricant or grease to work properly. Submersing this lubricant in the pool can result in the undesirable result of contaminating the pool water.
Finally, prior art transfer cask designs utilizing the retractable door assembly may not fit into the spent fuel pools of some nuclear power plants. This problem results because the rectangular compartments often have a large footprint which is necessitated by the presence of the retractable plates.